SIGNUM

The guard for signum requires its argument to be rational (real, in
ACL2(r)) number.

Signum is a Common Lisp function. See any Common Lisp
documentation for more information.

From ``Common Lisp the Language'' page 206, we see a definition of
signum in terms of abs. As explained elsewhere
(see abs), the guard for abs requires its argument to be a
rational (real, in ACL2(r)) number; hence, we make the same
restriction for signum.